Piezoelectric resonators are well known in the art. Piezoelectric resonators are electronic elements used to make a frequency stable and selectable. Piezoelectric resonators are widely used in various kinds of electronic equipment including communication systems, intelligence sensors, precision guided munitions, cordless telephones, broadcast and television, satellite telecommunication, electronic clocks, digital instruments and so on. Piezoelectric resonators can also be used as sensors of temperature, pressure, and weight. The properties of the crystal resonator depend on the angles of cut. Metal electrodes are disposed upon the crystal wafer, which is mounted in a structure designed to hold the crystal wafer. This crystal and holder assembly is called a piezoelectric resonator. Piezoelectric crystal devices are used primarily for precise frequency control and timing. Quartz is the most widely used piezoelectric material. Quartz resonators are manufactured by cutting wafers from the mother crystal along precisely controlled directions with respect to the crystallographic axes. A quartz crystal acts as a stable mechanical resonator, which, by its piezoelectric behavior and high Q, determines the frequency generated in an oscillator circuit. Bulk-wave resonators are available in the frequency range from about 1 kHz to 200 MHz. Surface-acoustic wave (SAW) and shallow-bulk-acoustic-wave devices can be made to operate at well above 1 GHz.
A typical low noise oscillator uses a crystal resonator as the frequency-determining element. An understanding of the fundamental nature of acceleration sensitivity in crystal oscillators resides primarily in understanding the behavior of the crystal resonator. The driving factor behind the acceleration-induced frequency shift is shown to be deformation of the resonator. The deformation drives two effects: an essentially linear change in the frequency determining dimensions of the resonator and an essentially nonlinear effect of changing the velocity of the propagating wave.
In a similar fashion, the frequency of a piezoelectric resonator is also affected by other stresses that deform the resonator, including gravitational stress, vibration and shock, temperature, aging, thermal hysteresis and so on. Even the acceleration due to gravity produces measurable effects and the frequency of a piezoelectric resonator can shift significantly when turned upside down due to gravity. For example, when an oscillator using an AT-cut crystal is turned upside down, the frequency typically shifts about 4×10−9 and acceleration sensitivity of an AT-cut crystal is typically 2×10−9 g−1. The sensitivity is the same when the crystal is subjected to vibration, i.e., the time-varying acceleration due to the vibration modulates the frequency at the vibration frequency with amplitude of 2×10−9 g−1. In the frequency domain, the vibration sensitivity manifests itself as vibration-induced sidebands that appear at plus and minus the vibration frequency away from the carrier frequency. The acceleration sensitivity of SC-cut crystals can be made to be substantially less than that of comparably fabricated AT- or BT-Cut crystals.
The stresses caused by acceleration, vibration and shock are well-known to those skilled in the art. Periodic acceleration in the form of vibration can cause frequency modulation in piezoelectric resonators, and shock can cause a step frequency change in a piezoelectric resonator due to the piezoelectric resonator's acceleration sensitivity. Shock can also cause a permanent frequency change in a piezoelectric resonator if either the supporting structure or the electrodes are stressed beyond their elastic limits. If during shock the elastic limits in the crystal's support structure or in its electrodes are exceeded, the shock can produce a permanent frequency change. Crystal units made with chemically polished plates can withstand shocks in excess of 20,000 g. Such crystals have been successfully fired from howitzers, however this ability to withstand shock is not typical. Therefore the stresses caused by acceleration, vibration and shock and the consequent significant effects on piezoelectric frequency instability have caused prior art piezoelectric resonators to suffer from numerous disadvantages, limitations and shortcomings.
Thus, there has been a long-felt need to provide piezoelectric structures that reduce the undesirable and harmful effects of the stresses caused by acceleration, vibration and shock sensitivity and provide affordable and easy to produce piezoelectric resonators. As a general trend, each order of magnitude in precision of determining the device aspect ratio yields an order of magnitude reduction in the acceleration sensitivity being compensated. For example, if we try to build a length to width ratio of 2:1, making the device 2 mm +/−0.001 mm will be about 10 times better than building it 2 mm +/−0.01 mm, and so on. This inventor has observed that whenever the +/−tolerance is reduced by a factor of 10, the acceleration sensitivity aspect ratio compensation improves by about the same order of magnitude. In accordance with the present invention, achieving tolerances on the order of microns=0.001 mm should be feasible, as compared to the 1 mm tolerance of the prior art. The structures of the present invention provide reliable piezoelectric restraint mechanisms that substantially reduce the ill effects of acceleration sensitivity through an increasingly rigid and precise orientation system. The innovative piezoelectric resonator stress relief apparatus and systems of the present invention resolve the long-standing disadvantages, limitations and shortcomings of prior art acceleration sensitive resonators by stacking a number of rigid plates, layers and spring cushions around the piezoelectric resonator by means of a mounting structure that provides in-plane stress relief and precise definition of the mounting plane. The piezoelectric resonator stress relief apparatus and structures of the present invention can essentially eliminate any deformation or may include precisely defined features to tailor the allowed deformation of the piezoelectric resonator, without suffering from the long-standing disadvantages, limitations and shortcomings of prior art acceleration sensitive resonators.